Exhaust gas turbochargers are used in some applications for increasing the power output of engines. The volume flow of exhaust gas drives a turbine connected via a shaft to a compressor which compresses the intake air. The compression ratio is a function of the volume flow of the gas passing through the turbine. The exhaust gas turbocharger in existing approaches is designed so that high compression occurs, even at low gas flow rates. So that compression ratios and turbine rotational speeds that could damage the engine or exhaust gas turbocharger do not result at high gas throughput rates, a bypass around the turbine, known as a “waste gate,” is installed. A flap or valve is provided in this bypass which modifies the cross section of the bypass opening. In one known approach, the flap or valve is actuated by a linkage which is moved by an aneroid capsule. The diaphragm of the capsule is connected to the linkage. A spring in the capsule forces the diaphragm upward. The boost pressure which is supplied from the intake manifold via a hose pipe of the aneroid capsule acts against the spring force. At high boost pressures the boost pressure prevails, and the waste gate opens. This system acts as a mechanical-pneumatic regulation. Depending on the gas volume flow, specified boost pressures are established in the intake manifold. To enable the boost pressure to be varied independently from these physical factors, a timing valve is installed in the hose pipe leading to the aneroid capsule. The function of the boost pressure regulation is to actuate this timing valve in such a way that an intended boost pressure is established. As the timing ratio increases, increasingly more air is discharged from the hose pipe to the outside. As a result, the back pressure against the spring drops, the waste gate closes, and the boost pressure rises (see, for example, Bosch, Automotive Handbook, 3rd edition, pages 466–71).
It has been shown that other adjustment mechanisms for controlling the cross section of the bypass opening may also be used, such as actuation of the linkage of the flap by an electrical actuator. The pneumatic counter-coupling over the boost pressure, which makes the exhaust gas turbocharger inherently stable, is thus omitted. The pneumatic counter-coupling enlarges the cross section of the opening as the boost pressure increases, thereby preventing the turbine from overspeeding. Without pneumatic counter-coupling the exhaust gas turbocharger is co-coupling, and therefore unstable. Other adjustment mechanisms include, for example, a variable turbine geometry, a variable sliding turbine, or a valve in the waste gate which is moved by a servomotor. The counter-coupling characteristic is at least partially absent for these actuators as well. Therefore, there is a need for a boost pressure regulation which is universally applicable and which ensures the stability of the exhaust gas turbocharger.
Another consideration is that an electrical compressor is installed in series to improve the response characteristics of an exhaust gas turbocharger. This is set, for example, below a specified engine rotational speed when the driver requests acceleration (see, for example, U.S. Pat. No. 6,029,452). Boost pressure regulation should also be usable in such a system.